Secondly you avoid any possible damage should the piece of gear electrically fail while you're not there......

in an actual example a studio in my area left all their equipment On all the time.... there was a failure in his 2 inch tape recorder's Motor Drive Amp and the machine went into full torque on the supply motor. It ran that way for 3 days until the owner came in to do the next session!!!!

I've seen very poorly designed power amps put out DC directly to speakers, power supplys can fail putting out huge voltages and burn up ICs and whole cards....and so on, so it's just safer if you turn all the equipment Off when you're gonna leave for the day.

Of course if you're just leaving for a few of hours, I'd leave it On......

The idea of leaving gear turned ON all the time comes from the days of tube equipment. For tubes you've got to let them warm up and reach their normal operating temperature, and even then you'd have to wait until it quieted down. Rather than wait an hour for all this to happen (and the corresponding movement of the tube's filament during turn-on and turn-off) we just left the gear On all the time.

But todays gear doesn't need warming up (of course you should be running tape on your analog recorders as you're tidying up the studio and before you start to align the machine.... say running 10 minutes of tape should do it in the morning if the machine's been Off...)

Yes there is some minor stress on the Power Supplies, but it's really minor in a well built unit.

Of course you'll want to have a shut-down procedure so there aren't huge pops in the monitors as you turn it all off....